docs: add example for tweaking plugin options (#1025)

Author: gnulnx

docs/webpack.md

@@ -110,6 +110,24 @@ module.exports = {
 
 You will need to familiarize yourself with [webpack-chain's API](https://github.com/mozilla-neutrino/webpack-chain#getting-started) and [read some source code](https://github.com/vuejs/vue-cli/tree/dev/packages/%40vue/cli-service/lib/config) in order to understand how to leverage the full power of this option, but it gives you a more expressive and safer way to modify the webpack config than directly mutation values.
 
+For example, say you want to change the default location of index.html from */Users/username/proj/public/index.html* to */Users/username/proj/app/templates/index.html*.  By referencing [html-webpack-plugin](https://github.com/jantimon/html-webpack-plugin#options) you can see a list of options you can pass in. To change our template path we can pass in a new template path with the following config:
+
+``` js
+// vue.config.js
+module.exports = {
+  chainWebpack: config => {
+    config
+      .plugin('html')
+      .tap(args => {
+        args[0].template = '/Users/username/proj/app/templates/index.html'
+        return args
+      })
+  }
+}
+```
+
+You can confirm that this change has taken place by examining the vue webpack config with the **vue inspect** utility, which we will discuss next.
+
 ### Inspecting the Project's Webpack Config
 
 Since `@vue/cli-service` abstracts away the webpack config, it may be more difficult to understand what is included in the config, especially when you are trying to make tweaks yourself.